Switched mode power supplies can be used to create a direct current (DC) voltage from an alternating current (AC) voltage by switching current through an energy storage element such as a transformer. The duty cycle of the switching is controlled to regulate the output voltage to a desired level. Switched mode power supplies are generally efficient at heavier loads but less efficient at lighter loads. Two popular types of isolated switched mode power supplies are forward mode and flyback mode converters.
Flyback converters are common in AC voltage to DC voltage applications. A flyback converter is based on a flyback transformer that alternately builds up flux in the magnetic core and transfers energy to the output. When current is switched through the primary winding, the primary current in the transformer increases, storing energy within the transformer. When the switch is opened, the primary current in the transformer drops, inducing a voltage on the secondary winding. The secondary winding supplies current into the load. A controller varies the on- and off-times of a primary switch in series with the primary winding to regulate the output voltage to a desired level.
A quasi-resonant (QR) flyback converter is a variable switching frequency converter that detects transformer demagnetization and subsequent resonant “valleys” of the primary field effect transistor (FET) drain voltage to determine when the next switching cycle will begin. This operation is therefore a discontinuous conduction mode of operation with the added benefit that the turn-on of the primary FET is synchronized to a valley, which may reduce the power dissipation in the device during the turn-on transition. QR flyback converters may be used in off-line AC-DC consumer applications because they offer a cost effective, isolated conversion with reduced turn-on voltage of the primary FET. However known QR flyback control algorithms are less efficient at light loads, causing problems in meeting energy efficiency requirements specified by various governmental organizations.
The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.